(a) Field of the Invention
The present invention relates to a longitudinally effective piezoelectric element. More particularly, it relates to a longitudinally effective piezoelectric element exhibiting a giant displacement at low voltage and to a ceramic composition suitable for producing the longitudinally effective piezoelectric element.
Such type of longitudinally effective piezoelectric element is practically applied to a wire driving element of impact dot matrix printers, a positioning element of super precision main table for use in the production of semiconductors and other various elements. It is now expected to further develop in the future for accurate positioning elements and micro driving elements.
(b) Prior Art of the Invention
Conventionally, typical actuators are motors driven by electromagnetic force, systems converting rotation of electromagnetic motors to a linear motion through combination of gears or voice coils obtained by combining electromagnetic coils with springs. These actuators are widely used in machinery for high speed continuous rotation and positioning.
Recently, the demand for new actuators has rapidly increased mainly in the field of optical precision instruments and semiconductor elements. For example, the demand is in the working of optical instruments such as lasers and cameras, in the positioning of semiconductor production equipment, and in the adjustment of optical-pass length in optics and astronomy. The required precision has already reached to a level of 1 micron or less and the requirement for precision will become increasingly severe. In order to satisfy these requirements, conventional positioning devices utilizing electromagnetic motors are too complex in construction and controlling, and voice coils have also disadvantages in output and response.
General characteristics required for a novel actuator in place of the above conventional actuators are summarized as follows.
1 Large displacement, PA1 2. No or small hysterisis error, PA1 3. Quick responsivility, PA1 4. Driving ability under low energy, PA1 5. Large output, PA1 6. Slight influence by temperature variation, PA1 7. Small size and light weight, PA1 8. No deterioration during use.
Recently, attention has been rapidly focused on an electrostrictive actuator utilizing piezoelectric and/or electrostrictive properties of ceramics as a candidate for the novel actuator satisfying the above requirements.
The electrostrictive property sometimes includes, in a broad sense, a piezoelectric property in that ceramics generate strain when an electric field is applied to the ceramics. However, in a narrow sense, the electrostrictive property is defined as a property where the strain factor is proportional to the square of electric field strength. On the other hand, the piezoelectric property is defined as a property where the strain factor is proportional in a first order to electric field strength. Further, the ceramics having a piezoelectric property causes spontaneous polarization and hence exhibits a very high coercive field and polarization treatment must be carried out in order to use it for elements. The present invention defines the piezoelectric material having a coercive field in excess of 5 kV/cm as a hard material. On the other hand, the ceramics having an electrostrictive property have a coercive field of 0 kV/cm and causes no spontaneous polarization. Consequently, no polarization treatment is required. The present invention defines the ceramics as an electrostrictive material. In addition, an intermediate piezoelectric material between the hard material and the electrostrictive material, practically a piezoelectric material having a coercive field of approximately from 0 to 5 kV/cm, easily changes its direction of polarization depending upon the applied electric field. Such type of piezoelectric material has also no requirement for polarization treatment. The present invention defines such type of piezoelectric material as a soft material.
As mentioned above, the electrostrictive material is roughly classified into three groups, that is, (1) a hard material, (2) a soft material and (3) an electrostrictive material. Representative examples of electric field-strain curves of these materials are illustrated in FIGS. 1(a), 1(b) and 1(c).
As illustrated in the electric field-strain curves of FIGS. 1(a), 1(b) and 1(c), in the cases where the electric field is alternately applied to these materials, the samples initially cause shrinkage in appearance due to their residual polarization as compared with the state thereof in the electric field level of 0 kV/cm and then lead to a rapid expansion. The coercive field of the present invention is defined as a value of the electric field where the rapid expansion starts. The strain factor is a value represented by the displacement amount (.DELTA.l) and the length (l) of a sample in the direction of applied electric field when an electric field of 10 kv/cm is applied to the sample. The strain factor is indicated by .DELTA.l/l.times.100 (%).
At present a generally known electrostrictive material is: EQU Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3
and further includes ceramics based on: EQU (Pb,Ba)(Zr,Ti)O.sub.3
(hereinafter abbreviated as PBZT).
As to PBZT ceramics, K. M. Leung et al of HANEY WELL CO. already reported on a composition: EQU Pb.sub.0.73 Ba.sub.0.27) .sub.0.97 Bi.sub.0.02 Zr.sub.0.70 Ti.sub.0.30 O.sub.3
[Ferroelectric, vol 27, page 41-43(1980) ].
According to the report, the composition had an electrostrictive property and a coercive field of 0 kv/cm. Even though an electric field of 10 kv/cm was applied to them, the ceramics of the above composition exhibited a strain factor of only 0.06 %. The present inventors also confirmed the same result in the below described Example 40. According to the investigation of this inventors, the dielectoric permittivity is from 5000 to 6000.
The ceramics of such composition require high voltage in order to obtain a large displacement, also have a disadvantage of a large electrical consumption due to their high dielectric permittivity during drive in high-frequency electric field and hence are quite difficult to be used for actuators. Further, the ceramics of such composition exert a marked weight loss in a sintering process. The weight loss amounts to 10 %. The weight loss is assumed to depended on lead evaporation which causes severe problems in the industrial production.
On the other hand, Japanese Patent Laid-Open Publication 60-144984(1985) discloses a ceramic composition on the basis of PBZT+Pb-Ba-Bi-W where the content of W+Bi is 1.5 atom % or less. The composition can be applied to actuators utilizing a transverse piezoelectric effect.
The transverse piezoelectric effect refers to the effect utilizing displacement in the rectangular direction to the direction of applied electric field. The effect is used, for example, for bi-morph elements etc. On the other hand, the longitudinal piezoelectric effect refers to the effect utilizing displacement parallel to the direction of applied electric field. The effect is used for multilayered piezoelectric elements etc. as illustrated in the present invention. The ceramic composition disclosed in Japanese Patent Laid-Open Publication No. 60-144984(1985) is a hard material having a coercive field of exceeding 5 kV/cm. The composition must be subjected to polarization treatment prior to use as illustrated in the examples of the publication. As illustrated in Examples 41 and 42 described below, the ceramic composition exhibits a strain factor Of only 0.08 % even in the electric field of 10 kV/cm, and cannot be employed at all for the elements exerting a large displacement.
Many patents have been applied for ceramic compositions containing PBZT and various metal dopants. Several patents have also been applied for ceramic compositions containing alkali metals and/or alkali earth metals in place of lead. Any of these compositions, however, is used for elements or filters having a high dielectric permittivity and not characterized by a large displacement.
As to the longitudinally effective electrostrictive element, a multilayered type longitudinally effective piezoelectric element is marketed from NEC Corp. According to the catalog thereof, the element has a coercive field of 6 kV/cm. The strain factor is 0.09 % at an applied electric field of 15 kV/cm and about 0.06 % at an applied electric field of 10 kv/cm. These displacements are considerably small and hence a complex displacement magnifying mechanism or a high voltage generator is required. The element still has many disadvantages to be improved.
Other longitudinally effective electrostrictive elements have also been reported in papers etc. However, any of these elements has a similar strain factor to that of the above NEC's element.
As mentioned above, the longitudinally effective piezoelectric element having a large displacement has not yet been fully developed.